As described in German patent document 2,927,873 filed 11 July 1979 by H. Steinort and E. Hortmann, a standard tunnel-type batch-washing machine has a normally cylindrical drum centered on and rotatable about a horizontal axis and provided internally with a helicoidal wall or rib that is also centered on the drum axis. The wall defines below the axis a succession of separated compartments each adapted to hold a respective batch of wash along with an appropriate treatment liquid, normally either soapy water or rinse water although the invention could be used with other batch-type liquid-treatment machines. For washing action the drum is oscillated about its axis through substantially less than 360.degree. so that each batch just moves angularly back and forth and also limited longitudinally in its compartment with its treatment liquid. Periodically the drum is rotated through more than a full revolution so that each batch is stepped or indexed downstream into the next compartment where the treatment liquid might be changed and the next stage of the washing operation takes place. As described in German patent document 2,944,857 it is also known to provide the drum internally with longitudinally extending ribs to enhance the mechanical washing action.
The main problem with such machines is that their capacity is relatively limited, that is the batch size is quite small. The only way to increase this capacity is with a substantial increase in radius. Since, however, the batches use only the bottom half or third of the drum, it is necessary to increase the drum size enormously for only a modest increase in batch size.